You count back from the date you know they have the documents in their possession to continue processing.
The type of security check depends on what they asked for? Was it military info. they were requesting? As PMM said, that takes longer like ours did. We were specifically asked in the first week of Feb 2007 to provide more detailed info. about my husband's military service so we know that's what it was hung up on, and it took them a year until first week of Feb 2008 just to confirm he was a supply office clerk. I enquired a few times and was told his military service was "still under investigation".
If it was just a police record or something similar it would have been taken at face value and added to the file, but I wouldn't think further security checks would then go on beyond that unless the particular country or embassy has more involved checks than what our basic police and judicial checks did.